Integrated circuits are often formed on substrates, such as substrates of semiconducting material. Such substrates can hold as few as one or many as thousands of the integrated circuits. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Integrated circuits are subjected to many different tests and analyses during the fabrication cycle, to determine whether the materials and structures of the integrated circuits are formed correctly. Such tests typically include charge-voltage (QV) tests of the substrates on which the integrated circuits are formed.
A typical QV test runs in the following sequence:                (1) Move position A of the substrate under a corona gun,        (2) Deposit a charge Q at position A,        (3) Move position A under a voltage probe,        (4) Measure at least one of the surface voltage (V) and the surface photo voltage (SPV) at position A, and        (5) Repeat the steps as desired at additional positions.        
There are at least two problems with this method, which are generally referred to as lateral spreading and time trending. The first problem is lateral spreading of the deposited corona charge. The deposited charge in step (2) tends to spread out on the substrate surface before the surface voltage is measured in step (4). The degree of the spreading is especially severe when the surface electrical conductivity is appreciable and the deposited area is small (such as less than about one millimeter in diameter). Unfortunately, the degree of spreading is difficult to predict quantitatively, and generally depends upon several factors, such as:    (1) Type of dielectric. The degree of spreading is tends to be generally greater in high-k materials than it is in silicon oxides. In addition, silicon oxynitride materials can also exhibit significant spreading.    (2) Prior processing of the substrate. For example, a silicon oxynitride layer without a final anneal tends to exhibit a significant degree of charge spreading. In contrast, a final anneal on the same layer generally reduces the charge spreading significantly. In general, any process that alters the surface structure and chemistry of the substrate would also tend to affect the degree of the charge spreading. Such processes include any heating, light illumination, and plasma processes.    (3) Ambient environment. The degree of spreading also generally depends on the environment in which the substrate is disposed. For example, the humidity of the environment significantly affects the degree of charge spreading. When the relative humidity is greater than about sixty percent, charge spreading is so great that a reliable QV measurement is typically not possible. Airborne molecules in the environment other than water can also affect the degree of charge spreading.
The second problem is time trending. Here, the QV measurement trends with the queue time of the substrate, which is defined as the length of time between the formation of the dielectric and the QV measurement. Silicon oxide exhibits a relatively lesser degree of time trending, but silicon oxynitride and high-k materials tend to exhibit a relatively greater degree of time trending. Similar to the problem of charge spreading, time trending also degrades the repeatability of QV measurements.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.